The Earth's climate is undoubtedly changing, however the timescale, consequences and causal attribution remain the subject of significant debate and uncertainty. Detection of subtle indicators from a background of natural variability require measurements over a time-base of decades. This places severe demands on the instrumentation used—requiring measurements of sufficient, accuracy and sensitivity that can allow reliable judgements to be made decades apart. The SI and network of National Metrology Institutes was developed to address such requirements. However, ensuring SI traceability of sufficient accuracy can be established and maintained to instruments orbiting the Earth presents a significant new challenge to the metrology community. This paper highlights some key measurands and applications driving the uncertainty demand of the climate community in the solar reflective domain, e.g., Solar irradiances and Earth reflectances/radiances. It discusses how meeting these uncertainties facilitate significant improvement in the forecasting abilities of climate models. After discussing the current state-of-the-art it describes a new satellite mission, called TRUTHS, which enables, for the first time, high accuracy SI traceability to be established in-orbit. The direct-use of a “primary standard” and replication of the terrestrial traceability chain, extends the SI into space in effect realising a “metrology laboratory in space”.
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